Sheet materials used in making medical drapes, medical gowns and absorbent articles, such as diapers and sanitary napkins, must be both comfortable and substantially liquid impermeable. Manufacturing and use requirements for such products often demand that the sheet material also be strong and durable.
Infants and other incontinent individuals wear absorbent articles to receive and contain urine and other body exudates. Absorbent articles function both to contain the discharged materials and to isolate these materials from the body of the wearer and from the wearer's garments and bed clothing. Disposable absorbent articles having many different basic designs are known to the art. It is also known that the exterior of absorbent articles can be covered with a flexible, fluid and vapor impervious sheet to prevent any absorbed fluid from passing through the article and soiling adjacent articles such as clothing, bedding and the like. These outer covers, generally referred to as backsheets, are often constructed from fluid and vapor impervious films such as polyethylene.
While plastic films do an admirable job of containing liquids, they are not pleasing to the touch and they do not readily pass moisture vapor, which makes garments made with plastic films uncomfortable and irritating to the skin. Plastic films have been made more acceptable for apparel and personal care applications by creating micropores in the films to make breathable microporous films. In microporous films, moisture is transported through the films by way of small gaps or holes in the film. One notable microporous film composite is made from polytetrafluoroethylene that is adhered to a textile material with an adhesive, as disclosed in British Patent Application No. 2,024,100. Microporous films adhesively bonded to textile substrates have been used in a variety of apparel products, including absorbent articles, as disclosed in PCT Patent Publication Nos. WO 95/16562 and WO 96/39031.
Laminates of a microporous film and a fibrous textile substrate have a number of disadvantages, including that their manufacture requires a separate adhesive bonding step after the film is made, and that such laminates permit some seepage of fluids when used as the backsheet in an absorbent article. For example, when such microporous film laminates are used as a backsheet of a disposable diaper, the backsheet may permit the transmission of some urine through the pores in the backsheet when an infant wearing the diaper sits down. Liquid seepage through microporous film laminates is especially likely to occur when the microporous laminate is exposed to a fluid with a low surface tension, as for example when urine in a diaper is exposed to surfactants within the diaper itself.
When fluids seep through the pores of a microporous film, bacteria, viruses, and other microbes can pass through the film along with the fluids. Likewise, the passage of fluids through laminates made with microporous films, whether the fluids are liquid or gaseous, also increases the odors that emanate from such laminates. Microbial adsorbents have been added to some microporous films in an attempt to capture microbes passing through such films, as disclosed in PCT Patent Publication No. WO 96/39031. However, it is difficult to distribute microbial adsorbents throughout a microporous film in a manner that will adsorb all microbes seeping through the holes in the film. Likewise, microbial adsorbents are unlikely to prevent the passage of odors through the pores in a microporous film.
Moisture vapor permeable films comprised of polyether block copolymers, like the film disclosed in U.S. Pat. No. 4,493,870, have an advantage in medical apparel and personal care applications because such films are non-porous and therefore substantially impermeable to fluids, but they permit the passage of moisture vapor. U.S. Pat. No. 4,725,481 suggests that such films may be attached to a textile fabric by adhesive bonding or melt bonding. However, the cost of making such films and then bonding the films to fibrous textile substrates has been high relative to microporous film laminates. In addition, known moisture vapor permeable films like the films disclosed in U.S. Pat. Nos. 4,725,481 and 5,445,874 do not readily adhere to many common nonwoven substrate materials, such as polyolefin-based nonwoven materials, without the application of a separate adhesive.
PCT Patent Publication No. WO 95/16746 (assigned to E.I. duPont de Nemours & Company (hereinafter “DuPont”)) discloses a composition of a polyether block copolymer combined with a less costly thermoplastic homopolymer so as to make an overall film that is less costly, more heat sealable and more adherable to itself and other substrate materials. However, PCT Patent Publication No. WO 95/16746 does not disclose strong and durable composite sheets of thin breathable films that have been extruded directly onto fibrous substrates, nor does it disclose a method for making such composite sheets.
There is a need for a sheet material that acts as a barrier to fluids, yet is also highly permeable to moisture vapor. There is also a need for a sheet material that readily transmits moisture vapor, but significantly deters the passage of bacteria and odors associated with such fluids. There is a further need for such a moisture vapor permeable, fluid impermeable composite sheet material that is also durable, strong, and flexible enough to be used in absorbent articles, and can be produced in an economical fashion, i.e., without the use of adhesives to join the layers of the composite sheet in a separate step. Finally, there is a need for an absorbent article that incorporates such a moisture vapor permeable composite sheet in the article's backsheet, leg cuffs, waistshields, or other features.